This invention relates to a method and apparatus for measuring the amount by which glare blocks a person's vision as a result of disperson of light as perceived in the subject's eye.
When a person looks at a light source, e.g., an oncoming automobile headlight, the incoming light rays can be dispersed through the eye due to various eye conditions such as cataracts, intraocular lens implants, the presence of a contact lens or spectacle lenses, and anatomic changes of the corneal surface. In dark surroundings this dispersion can substantially interfere with the eye's ability to detect and resolve other objects, e.g., road boundaries or street signs. A point source of light at a distance may cause what may be referred to as "blocking glare."
The problem of blocking glare may be especially severe in subjects having cataracts. An abnormally high degree of vision impairment arising from blocking glare can indicate the onset of cataract formation.
The blocking effect of glare has been detected by measuring the resulting reduction in visual acuity. Variations on the so-called Snellen method, for example, entail asking the subject to read lines on a conventional eye chart while a light source is directed into his eyes. Similarly, the Miller-Nadler method calls for the subject to identify the orientation of an object (e.g., the letter "C") while a peripheral light source simulates daytime glare. The Terry method calls for the subject to read an eye chart while subjected to a a glare source. The Ginzburg method generally uses a contrast-sensitivity approach.
The use of diminution of visual acuity as an indicator of the impact of glare, however, is necessarily dependent on the subject's acuity generally, which may vary independently of glare and is concomitantly of less reliabilty than is desirable.